Arizona Cardinals QB Carson Palmer suffered a knee injury against the St. Louis Rams that caused him to miss the rest of the game. The Cardinals are afraid that their star QB – who is 6-0 as a starter this season – may have torn his ACL.

Cardinals head coach Bruce Arians was unsure about the severity of the injury. “We don’t know anything about Carson’s injury or the length or whatever it is yet,” Arians said. “It is obviously a knee, but he will be evaluated and get an MRI and everything, and we’ll know something more hopefully by Tuesday.”

Palmer was injured on a sack by Rams safety Mark Barron with about 11:21 left in the fourth quarter Sunday, just two days after he signed a three-year contract extension worth a reported $50 million, including $20.5 million guaranteed. Barron hit Palmer but didn’t bring him down. However, Palmer went to the turf a moment later and lay on the field while he was attended to by team trainers and doctor. While he was walking off the field, he appeared to stumble and had to be held up. He was then carted off the field.

Palmer has missed three games earlier this season with a damaged nerve in his throwing shoulder. The Cardinals won two of those three games with Drew Stanton starting at quarterback.